A prerequisite for normal spermatogenesis is the coordination of Leydig cell function with the needs of the spermatogenic phase of the seminiferous epithelium. This is achieved through the actio of gonadotropins, and a reciprocal transfer of information between Leydig cells and Sertoli cells. It is also now clear that a local modulation of Leydig and Sertoli cells, possibly independent of LH and FSH, occurs within the testis and that these paracrine interactions are important in regulating sperm production under normal conditions, or as part of a local compensatory mechanism which operates when Leydig cell function needs to be supported. The proposed studies focus on the regulation of Leydig cell function by Sertoli cells. Our first specific aim is the purification and characterization of the Sertoli cell secreted protein which stimulus Leydig cell steroidogenesis. Generation of antibodies, partial amino acid sequencing, and molecular cloning of the bioactive protein will permit us to determine its structure/function relationship[ as well as its relationship to other known protein(s). Identification of the receptor system through which this Sertoli cell protein acts on Leydig cells, and of the step(s) in the steroid biosynthetic pathway which are regulated by this protein is our second specific aim. Part of these studies will be to determine whether and how this protein modulates Leydig cell responsiveness to LH. In view of our recent findings that the polypeptide diazepam binding inhibitor (DB or endozephine) is a paracrine/autocrine regulator of Leydig cell proliferation and steroidogenesis, our third specific aim focuses on the understudying of the mechanism of action of DB on Leydig cells.